Transit-time and temperature control the spatial patterns of aerobic respiration and denitrification in the riparian zone

Abstract

During the flow of stream water from losing reaches through aquifer sediments, aerobic and anaerobic respiration (denitrification) can deplete dissolved oxygen and nitrate (NO3− ), impacting water quality in the floodplain and downstream gaining reaches. Such processes, which vary in time with short and long-term changes in stream flow and temperature, need to be assessed at the stream corridor scale to fully capture their effects on net turnover, but this has rarely been done. To address this gap, we combine a fully-integrated 3D transient numerical flow model with temperature-dependent reactive transport along advective subsurface flow paths to assess aerobic and anaerobic respiration dynamics at the stream corridor scale in a predominantly − losing stream. Our results suggest that given carbon availability (as an electron donor), complete NO3 removal occurred further away from the stream after complete oxygen depletion and was relatively insensitive to variations in temperature and transit-times. Conversely, transit-times and oxygen concentrations constrained nitrate removal along short hyporheic flow paths. Even under limited carbon availability and low-temperatures, − NO3 removal fractionsE (RNO3) will be greater at locations further from the stream than along shorter hyporheic flow pathsE (RNO3 = 0.4E and RNO3 = 0.1, respectively). With increasing temperature, the relative effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream decreased. The study highlights the importance of seasonal variations of stream flow and temperature for water quality at the stream-corridor scale. It also provides an adaptive framework to assess and quantify reach-scale biogeochemical turnover around dynamic streams. Plain Language Summary Nitrate pollution is a widespread problem in many catchments with intense agricultural activities. Denitrification is a redox process that removes nitrate from the aquatic system via its transformation to nitrogen gas. Denitrification is difficult to assess at larger scales since it depends on multiple factors, such as solute concentrations, temperature variations, and also the time that water resides in the subsurface, where reactions can take place. To evaluate how these factors can influence denitrification, we employed a coupled modeling approach representing the riparian zone of a 4th order stream in central Germany. We found that temperature variations strongly regulate the process and that during the winter the aerobic (oxygen rich) zone around the stream expands, which further inhibits denitrification in the near stream groundwater. However, even in the winter denitrification occurs, but at larger distance from the stream where oxygen has been depleted sufficiently. With increasing temperature, the influence of other factors on denitrification and on the redox zonation around the stream decreases. Coupled numerical models can provide further insights into the occurrence and interrelations of the multiple processes controlling water quality patterns in river corridors.